1. Field of the Invention
This invention relates to a device for inspecting whether a pixel portion properly operates prior to forming an EL (electroluminescence) element in a light-emitting device in which the EL element is formed on a substrate and to a method of inspection. The EL (electroluminescent) devices referred to in this specification include triplet-based light emission devices and/or singlet-based light emission devices, for example. More particularly, the invention relates to a device for inspecting whether the pixel portion properly operates prior to forming an EL element in a light-emitting device that uses a semiconductor element (using a thin semiconductor film), to a method of inspection, to a method of fabricating the light-emitting device that incorporates the inspection method in one of the fabrication steps and to the light-emitting device fabricated by using the above fabrication method.
The EL element according to this invention has a structure in which an EL layer is sandwiched between a pair of electrodes. The EL layer stands for a layer containing an organic compound that emits fluorescent light or phosphorescent light upon the application of an electric field.
The light-emitting device to be inspected by the inspection device of this invention stands for an image display device or a light-emitting device using an EL element. Further, the light-emitting device encompasses all of those modules in which a connector such as an anisotropic electrically conducting film (FPC: flexible printed circuit), a TAB (tape automated bonding) tape or a TCP (tape carrier package) is attached to the EL element, modules in which a printed wiring board is provided at an end of the TAB tape or the TCP, or the modules in which an IC (integrated circuit) is directly mounted on the EL element by a COG (chip-on-glass) system.
2. Description of the Prior Art
In recent years, technology has been greatly advanced concerning forming TFTs (thin-film transistors) on a substrate, and attempts have been made to apply the technology to the active matrix display devices (light-emitting devices). In particular, the TFT using a polysilicon film exhibits a field-effect mobility (also called mobility) which is higher than that of the conventional TFT using an amorphous silicon film, and makes it possible to accomplish a high-speed operation. This makes it possible to control the pixels that had been controlled by a drive circuit outside the substrate by using a drive circuit formed on the same substrate as the pixels.
In the active matrix light-emitting device, various circuits and elements are formed on the same substrate to obtain various advantages such as decreasing the cost of production, decreasing the size of the electro-optical device, increasing the yield and decreasing the throughput.
Further, study has been vigorously forwarded concerning the active matrix light-emitting device (inclusive of EL display) having an EL element as a self-light-emitting element. The light-emitting device is also called an organic EL display (OELD) or an organic light-emitting diode (OLED).
The EL element possessed by the light-emitting device has a structure in which the EL layer of an organic compound is sandwiched between a pair of electrodes (cathode and anode). Here, however, the EL layer usually has a laminated-layer structure. A representative example may be a laminated-layer structure of “positive hole-transporting layer/light-emitting layer/electron-transporting layer” proposed by Tang et al. of Codak Eastman Co. This structure features a very high light-emitting efficiency. Most of the light-emitting devices that have now been studied and developed are employing this structure.
There may be further employed a structure in which the positive hole-injection layer/positive hole-transporting layer/light-emitting layer/electron-transporting layer or positive hole-injection layer/positive hole-transporting layer/light-emitting layer/electron-transporting layer/electron injection layer are laminated on the anode in order mentioned. The light-emitting layer may be doped with a fluorescent pigment.
In this specification, the layers provided between the cathode and the anode are all called EL layers. Therefore, the above positive hole-injection layer, positive hole-transporting layer, light-emitting layer, electron-transporting layer and electron injection layer all pertain to the EL layers.
A predetermined voltage is applied from a pair of electrodes to the EL layer of the above structure, whereby the carriers are recombined in the light-emitting layer to emit light. In this specification, a light-emitting element formed by the anode, EL layer and cathode is called EL element.
The EL layer possessed by the EL element is deteriorated by heat, light, moisture and oxygen. In fabricating the active matrix light-emitting device, therefore, the EL element is formed after the wiring and TFT are formed in the pixel portion.
After the EL element is formed, the substrate (EL panel) on which the EL element is provided and a cover member are stuck and sealed (packaged) together with a sealing member in a manner that the EL element is not exposed to the external air.
After the air-tightness is heightened by the treatment such as packaging, a connector (FPC, TAB, etc.) is attached for connecting the terminals drawn from the element or the circuit formed on the substrate to the external signal terminals, thereby to complete the active matrix light-emitting device.
In the active matrix light-emitting device, however, a predetermined voltage (current flowing into the EL layer) applied to the EL layer from the pair of electrodes of the EL element is controlled by a transistor provided in each of the pixels. Therefore, if some trouble occurs such as failure of the function of the transistor in the pixel portion, break or short-circuiting of the wiring, the predetermined voltage (current) is no longer applied to the EL layer possessed by the EL element. In such a case, the pixel no longer displays a desired gradation.
Even when the wiring or the transistor for controlling the emission of light from the EL element is defective in the pixel portion, however, it is not possible to make sure the presence of the defect until the light-emitting device is completed and is really used to make a display. In order to make a distinction from the acceptable products by inspection, therefore, the EL element must be completed though it may include a pixel portion that does not serve as a completed product, the packaging must be effected, and the connector must be attached to complete it as the light-emitting device. In this case, the step of forming the EL element, the step of packaging and the step of attaching the connector are wasted, resulting in a loss of time and cost. Even when the EL panel is formed by using a multi-chamfered substrate, the step of packaging and the step of attaching the connector are wasted, similarly, resulting in the loss of time and cost.
In the active matrix liquid crystal displays that are mass-produced earlier than the active matrix light-emitting devices, it has been done to form the wiring and TFT in the pixel portion prior to completing the liquid crystal display by introducing the liquid crystals into between the two substrates, to electrically charge the capacitors possessed by the pixels, and to measure the amount of electric charge for each of the pixels to make sure the presence of defects in the pixel portions.
In the active matrix light-emitting devices, however, not less than two TFTs are generally included in each pixel. One electrode (pixel electrode) and the capacitor in the EL element are often connected together with the transistors sandwiched therebetween. In this case, measurement of the amount of electric charge stored in the capacitor does not help make sure if the wiring and transistor connected between the capacitor and the pixel electrode are defective. In the case of the light-emitting device, further, an electric current must be supplied to the EL element and, hence, it is necessary to measure the electric current that flows.
It has been urged to establish the method of inspecting whether the wiring and transistor in the pixel portion are defective or, in other words, whether a predetermined voltage can be applied (or, whether a predetermined current can be supplied) to the pixel electrode of the EL element of each pixel prior to completing the light-emitting device in a process toward mass-producing the active matrix light-emitting devices.